The present invention relates generally to the field of digital signal processing and more particularly to systems and methods for calibrating multi-channel subsystems.
In the field of analog and digital communications, a signal is commonly defined as anytime-varying or spatial-varying quantity that conveys information (e.g., an energy signal or power signal). Presently, there exists a wide variety of devices that are designed to process analog and/or digital signals in some capacity.
To enhance its overall performance, signal processing systems often rely upon multi-channel subsystems. As defined herein, a multi-channel subsystem relates to a plurality of individual signal processing elements which are combined together. As can be appreciated, the use of a combination of signal processing elements, rather than a single element or device, serves to improve the overall dynamic range and resolution of the signal processing system in the particular domain in which it senses or samples (e.g., time, frequency and angle space, etc.) and, as such, is highly useful in the art.
Examples of well-known multi-channel subsystems include: (i) analog-to-digital conversion systems which comprise a plurality of individual analog-to-digital converters (ADCs) which are combined to yield a signal processing system with a higher sample frequency and improved amplitude resolution; (ii) antenna arrays which comprise multiple antenna elements which are combined to yield a system with improved spatial angle resolution and dynamic range; and (iii) sub-band sensor systems which comprise multiple individual components that handle adjacent frequency bands, the components being combined to cover a wideband of input signals.
The performance of signal processing systems which rely upon multi-channel subsystems is contingent upon multiple factors.
As a first factor, the performance of signal processing systems which rely upon multi-channel subsystems is contingent upon the individual performance of each element, or device, within the multi-channel subsystem. For example, the performance of an analog-to-digital conversion system of the type described above is largely dependent upon the individual performance characteristics of each analog-to-digital converter within the system. Some of the factors that often influence the performance of each signal processing element include, among other things: (i) noise (i.e., broadband signals which are generated by various environmental effects and/or man-made sources); (ii) nonlinear distortion products (e.g., harmonics, intermods, etc.); (iii) interference signals (either environmental or man-made); and (iv) sampling rate (i.e., the rate at which an analog signal is sampled).
Many of the aforementioned factors that influence the performance of each signal processing element are often treated using a non-linear equalizer (NLEQ) digital signal processor at its back end. Examples of NLEQ digital signal processors which are well known in the art are disclosed in U.S. Pat. No. 6,639,537 to G. M. Raz (hereinafter the '537 patent), U.S. Pat. No. 7,173,555 to G. M. Raz (hereinafter the '555 patent) and U.S. Patent Application No. 2006/0133470 to G. M. Raz et al. (hereinafter the '470 application), all of said disclosures being incorporated herein by reference.
As a second factor, the performance of signal processing systems which rely upon multi-channel subsystems is contingent upon the quality of the calibration of each element relative to one another. In particular, when the relationship between the individual elements of a multi-channel subsystem is improperly calibrated (e.g., with respect to their relative sample times, frequencies or locations), the performance of the multi-channel subsystem is significantly compromised, which is highly undesirable.
Traditionally, multi-channel subsystems are calibrated using single-channel correction means. Specifically, multi-channel subsystems are typically calibrated by either (i) improving the individual performance characteristics associated with each element (e.g., with respect to noise and nonlinear distortion), as described in detail above, and/or (ii) independently calibrating each element relative to a particular standard. However, it has been found that since both of the aforementioned processes relate solely to single-channel corrections within a multi-channel subsystem, significant relative calibration errors often remain between the plurality of elements, which is highly undesirable.
Referring now to FIGS. 1(a)-(f), there is shown a series of measured graphical representations which are useful in illustrating the inherent problems associated with the use of single-channel correction means for calibrating multi-channel systems. In FIG. 1(a), there is shown a graphical representation of a multi-channel digital output signal produced by a conventional high speed interleaved ADC in response to a two tone input signal 1. As can be seen, the interleaved ADC is responsible for producing a number of sizable distortions D which significantly limit the overall dynamic range of the system, which is highly undesirable.
As noted above, distortion products inherently introduced into the output signal of a multi-channel subsystem are often treated through the independent equalization of each individual sub-channel using well-known digital signal filtering techniques. Accordingly, as seen in FIGS. 1(b) and 1(c), the first step in the single-channel calibration process is to de-interleave (i.e., separate) the multi-channel, or complete-channel, output signal into its individual sub-channels x1, as shown in FIG. 1(b), and x2, as shown in FIG. 1(c). As can be seen, each of sub-channels x1 and x2 includes sizable and unique distortion products D along with two tone signal 1.
Accordingly, each of sub-channels x1 and x2 is independently equalized using well-known digital signal filtering techniques to yield corresponding equalized sub-channels x1′, as shown in FIG. 1(d), and x2′, as shown in FIG. 1(e). As can be seen, the equalization of sub-channels x1′ and x2′ serves to significantly reduce, if not eliminate, the presence of all distortion products D therein. However, it should be noted that when sub-channels x1′ and x2′ are re-interleaved (i.e., combined) to yield an equalized, multi-channel output signal y, as shown in FIG. 1(f), a new non-harmonic distortion N is produced as a direct result of relative calibration errors and other signal discrepancies between the multiple elements, this particular type of distortion being commonly referred to as an interleaved distortion in the art. As can be seen, interleaved distortion product N significantly reduces the dynamic range of the interleaved ADC, which is highly undesirable.